In an LTE (Long Term Evolution) mobile communication system, it is necessary for a handover source radio base station to designate Target ID for identifying a handover destination candidate radio base station eNB/HeNB in an HO Required message to be transmitted to a mobile switching center MME (Mobility Management Entity).
As illustrated in FIG. 8, an information element “Target ID” included in HO Required in an Intra-LTE handover includes an information element “Global eNB ID” and an information element “Selected TAI”.
Furthermore, as illustrated in FIG. 8, one of Macro eNB ID (identification information of a radio base station eNB) of 20 bit length and Home eNB ID (identification information of a radio base station HeNB) of 28 bit length is set in an information element “eNB ID” included in the information element “Global eNB ID”.
Furthermore, in an LTE-Rel.8 mobile communication system, as illustrated in FIG. 9, since a mobile station UE is configured to transmit Measurement Report which includes only PCI (Physical Cell ID) (Target PCI) of a handover destination candidate cell, that is, does not include identification information (Global eNB ID) of the handover destination candidate radio base station or identification information (TAI: Tracking Area ID) of a handover destination candidate tracking area, it is necessary for a handover source radio base station eNB#1 to extract the identification information (Global eNB ID) of the handover destination candidate radio base station or the identification information (TAI) of the handover destination candidate tracking area based on the PCI with reference to NRT (Neighbor Relation Table).
Therefore, it is not possible for the mobile station UE to perform an S1 handover with respect to a radio base station eNB/HeNB (a radio base station eNB#2 in the example of FIG. 9) not registered in the NRT of the handover source radio base station eNB#1.
Meanwhile, in an LTE-Rel.9 mobile communication system, since it is possible for a mobile station UE to transmit Measurement Report including E-CGI (E-UTRAN Cell Global Identity) and TAI related to a handover destination candidate cell, it is possible for a handover source radio base station S-eNB to acquire Global eNB ID and TAI which constitute Target ID for identifying a handover destination candidate radio base station not registered in the NRT.
FIG. 10 illustrates an example of an information element included in the information element “E-CGI” included in the Measurement Report.
Therefore, it is possible for the handover source radio base station to also transmit the HO Required message including the Target ID to the handover destination candidate radio base station eNB/HeNB not registered in the NRT or the handover destination candidate radio base station HeNB subordinate to a gateway device HeNB-GW, but it is not possible to determine whether the handover destination candidate radio base station included in the Measurement Report transmitted by the mobile station UE is the radio base station eNB or the radio base station HeNB.
That is, the E-CGI included in the Measurement Report transmitted by the mobile station UE does not include information for determining the type (eNB/HeNB) of the handover destination candidate radio base station.
In addition, when a lower 28 bit of the E-CGI includes Cell ID (corresponds to Cell Identity set in the information element “Cell Identity” illustrated in FIG. 10) and the handover destination candidate radio base station is the radio base station HeNB, the Cell ID includes Home eNB ID. When the handover destination candidate radio base station is the radio base station eNB, the Cell ID includes Macro eNB ID of 20 bit length and 8-bit identification information of a cell subordinate to the radio base station HeNB.
Furthermore, the mobile switching center MME searches for whether an entry coinciding with the Global eNB ID set in the information element “Global eNB ID” in the HO Required message received from the handover source radio base station is registered.
Here, when the entry has not been registered and the length of the information element “eNB ID” in the information element “Global eNB ID” is 20 bits, the mobile switching center MME transmits HO Failure to the handover source radio base station.
Meanwhile, when the entry has not been registered and the length of the information element “eNB ID” in the information element “Global eNB ID” is 28 bits, the mobile switching center MME searches for whether a gateway device HeNB-GW having TAI coinciding with the TAI set in the information element “Selected TAI” in the HO Required received from the handover source radio base station is registered.
When the gateway device HeNB-GW is registered, the mobile switching center MME transmits HO Request including the TAI to the gateway device HeNB-GW. When the gateway device HeNB-GW has not been registered, the mobile switching center MME transmits HO Failure to the handover source radio base station.
However, in the above-mentioned mobile communication system, when the S1 handover is performed, since it is not possible for the handover source radio base station to determine the type (eNB/HeNB) of the handover destination candidate radio base station based on the E-CGI included in the Measurement Report transmitted by the mobile station UE, there is a problem that it is not possible to decide the length (20 bits or 28 bits) of the information element “Global eNB ID” in the information element “Target ID” in the HO Required.